The present invention is in the field of transducers, and more particularly relates to high resolution, high accuracy force transducers.
One prior art form of force or weight sensing utilizes a feedback approach using a movable coil in a fixed magnetic field. The coil is movable along a sensing axis and is driven by a current sufficient to maintain a fixed position along the sensing axis. In this configuration, the coil drive current provides a measure of forces applied to displace that coil. While this approach is generally effective, the force sensing configuration is relatively complex and correspondingly expensive.
Another form in the prior art is a strain gage load cell. In this form, however, the accuracy of the load cell is limited by hysteresis and creep of the strain gage sensor material, as well as that of the bonding material for the sensor.
Yet another force sensing approach utilizes a variable capacitance type load cell, wherein a pair of opposed, substantially parallel, conductive plates are coupled so that the force-to-be-measured causes a separation of those opposed conductor plates in a manner which is proportional to an applied force. While in principle this approach is satisfactory, there are no straight forward assemblies known in the prior art which adequately maintain the sensing plates parallel over a range of forces. Moreover, the capacitance can be affected by fringe effects, the ambient environment including electromagnetic interference, temperature, humidity and variations in materials due to aging and creep.
Accordingly, it is an object of this invention to provide an improved force sensor.
It is another object to provide an improved variable capacitance force sensor free of the effects of spurious variations in capacitance.